Allllals of Claciology 7
© International
1985
Glaciological Society
ASSESSING LABORATORY PROCEDURES FOR THE DECONTAMINATION
OF POLAR SNOW OR ICE SAMPLES FOR THE ANAL VSIS OF TOXIC
METALS AND METALLOIDS
by
Claude F. Boutron and
Fran~oi s e
M. Batifol
(Laboratoire de Glaciologie et Geophysique de I'Environnement du C.N .R .S., B.P. 96 ,
38402 St.-Martin-d'Heres Cedex, France)
ABSTRACT
Most, if not all, Antarctic and Greenland snow and
ice samples to be analyzed for toxic metals and metalloids
such as Pb, Hg, Sb, Cd, Ag, Se, As, Cu and Zn become
more or less contaminated by these elements on their
outsides, mainly during field collection. We assess here the
various procedures which have been developed to try to
decontaminate the samples. They include both mechanical
and rinsing techniques. It appears that it is essential, but
very difficult, to establish clearly the efficiency of these
procedures. This can however be done by determining the
geometry of contamination of the analyzed samples and by
careful
evaluating
procedural
blanks
carefully.
Such
evaluation has been achieved at present only for mechanical
procedures and for a few metals.
I. INTRODUCTION
At the present time, great efforts are being made to
determine the occurrence of toxic metals and metalloids
such as Pb, Hg, Sb, Cd, Ag, Se, As, Cu and Zn in the
stratified snow and ice deposited in the central areas of the
Greenland and Antarctic ice sheets in order to obtain
historical records of the atmospheric concentrations of these
elements in the remote polar areas of both hemispheres (see,
for instance, Murozumi and others 1969, Ng and Patterson
1981, Boutron 1982). Concentrations to be measured are
unfortunately very low, in the range from 10- 10 to 10- 14 g
g-1, SO that it is extremely difficult to collect the samples
in the field and to analyze them for toxic metals and
metalloids in the laboratory without introducing an artifact
contamination . Many published data are th en erroneous b y
up to several orders of magnitude (Wolff and Peel 1985[bJ,
Boutron in press).
In this paper, we shall assess critically the various
procedures
which
have
been
developed
in
several
laboratories in the attempt to remove contaminants from
polar snow and ice samples intended for analysis for toxic
metals and metalloids . This is a critical point since it
appears that it is almost impossible to collect polar snow or
ice samples in the field without introducing surface
contamination for toxic metals and metalloids. The severity
of this outside contamination varies greatly with the
cleanliness of the field sampling procedures (Table I). It
ranges from enormous contamination in deep ice cores
thermally or electromechanically drilled in holes filled with
a stabilizing fluid (Ng and Patterson 1981) to very slight
yet significant contamination in shallow samples collected
from the walls of hand-dug pits by forcing acid-cleaned
plastic tubes into the snow.
suitable for both snow and ice samples . It involves the
mechanical chiselling of veneers of snow or ice in sequence
from the outside toward the centre of each sample. Each
veneer subsample and the remaining inner sample are then
analyzed separately. This procedure was developed initially
for the analysis of highly contaminated , deep ice-core
sections 10 to 12 cm in diameter, which were obtained by
electromechanical drilling in a fluid-filled hole in Greenland
(Hansen and Langway 1966) and by thermal drilling in a
non-fluid-filled hole in Antarctica (Ng and Patterson 1981).
Later, the procedure described by Ng and Patterson was
improved
to
allow
the
decontamination
of slightly
contaminated, shallow snow cores, 7.5 cm in diameter,
which were obtained by hand drilling with an acid-cleaned
polycarbonate auger in Antarctica (Boutron and Patterson
1983) and of moderately contaminated, large-size blocks of
prehistoric surface blue ice collected at an Antarctic coastal
ablation area (Boutron and Patterson 1983, Boutron and
others 1984).
As an illustration, the improved procedure used for
the shallow snow cores (Boutron and Patterson 1983) is as
follows. All equipment and clothing used throughout is
acid-cleaned. The snow-core sample is allowed to slide out
of the plastic tube in which it was collected, and is then
placed directly on a polyethylene sheet inside a cooled,
double-walled, conventional polyethylene tray. This tray is
located inside an ultra-clean laboratory flushed with filtered
air (Patterson and Settle 1976). The operators wear, over
their clean-room clothing, a large polyethylene bag and
shoulder-length polyethylene gloves. The first veneer layer
(about 0.5 to I cm thick) is then chiselled using a
stainless-steel chisel by making successive shallow chord
shavings from one end of the core section to the other
along the side; the ends of the core are also shaved. The
snow chips are allowed to fall directly into a specially
designed quartz tray standing alongside the core. The shaved
core is then transferred to a fresh clean part of the
working area, and a second veneer layer is then remo ved
similarly after putting on new acid-cleaned polyeth ylene
gloves and cleaning the chisel and the quartz tray once
again with ultrapure acids. The procedure is repeated until
the remaining snow core is 3 to 4 cm in diameter.
It should be noted that several other investigators have
tried to decontaminate snow or ice samples by mechanically
removing the outside of their samples (see, for instance,
Appelquist and others (1978) and Landy and Peel (1981».
However their procedures were unrefined and were much
less carefully controlled: they probably did not avoid
transfer, by entrainment, of surface contaminants towards
the cleaner, inner layers.
2.2
2. MECHANICAL DECONTAMINATION TECHNIQUES
2.1.
Decontamination by chiselling veneers of snow or
ice in progression from the outside to the inside of the
sample
Patterson and his co-workers at the California Institute
of Technology have developed a decontamination procedure
Direct subsampling using a sub-corer
Wolff, Peel and their co-workers at the British
Antarctic Survey have developed a mechanical subsampling
procedure suitable for firn samples only. This procedure has
been used for the decontamination of shallow firn cores
hand-drilled with an aluminium drill in the Antarctic
Peninsula and of surface snow samples collected directly
7
Boutron and Batilol:
Cleaning ice cores lor heavy metal analysis
TABLE I. ANTICIPATED CONTAMINATION FOR TOXIC METALS AND METALLOIDS IN
POLAR SNOW AND ICE SAMPLES ACCORDING TO THE MAIN FIELD SAMPLING
PROCEDURES USED
Type of sample
I. RECENT SNOW
Direct collection in
acid-cleaned plastic
containers from pits
Expected contamination for
toxic metals and metalloids
References
very small, however often
significant
highly dependent on
cleanliness of equipment
Herron and others
(1977)
Boutron (1982)
Hand-operated acidcleaned all-plastic
auger
small, however often
significant
highly dependent on
cleanliness of equipment
Boutron and
Patterson (1983)
Hand-operated
aluminium auger
medium
highly dependent on
cleanliness of equipment and drill
constitution
Conventional SIPREtype auger
11. ANCIENT ICE
Surface collection
in blue-ice coastal
areas
very high
Peel and Wolff
(J 982)
British Antarctic
Survey (1982)
Boutron (1979)
very small to medium
highly dependent on Murozumi and
sampling procedure
others (1969)
Boutron and
others (1984)
Thermal or electromechanical drilling in
a non-fluid-filled hole
very high
penetration to centre
of cores highly dependent on quality
of cores
Thermal or electromechanical drilling in
a fluid-filled hole
extremely high
penetration to centre
of cores highly dependent on quality
of cores
into acrylic tubes (British Antarctic Survey 1982, Peel and
Wolff 1982, Wolff and Peel 1985[a)).
The procedure (Wolff and Peel 1985[a]) can be
summarized as follows. One end of the core sample is first
discarded, either by cutting across the core using an arched
teflon slicing tool, which is shaped so that the section to be
subsampled is not touched b"y the tool, or by snapping the
core whilst it straddles two pre-cleaned acrylic tubes. The
core is then stood upright, and subsamples (0.8 to 3.7 cm
in diameter) are taken at various distances from the centre
of the core by pushing acid-cleaned, small-size teflon
sub-corers vertically into the core using, if necessary, a
polyethylene mallet. The procedure takes place inside a
laminar-flow clean bench placed in a conventional cold
room. The investigators wear acid-cleaned polyethylene
gloves.
2.3. Checking the integrity of the central parts of the
samples so obtained
It is absolutely fundamental to determine clearly
whether the most central parts of the samples obtained by
the procedures described in previous sections are free of
contamination or not. Such contamination could have
resulted either from diffusion of outside contamination
during field sampling or sample storage or from entrainment
and handling during the decontamination procedure itself.
This evaluation can be done only from profiles showing, for
each individual sample and each investigated metal or
metalloid, the concentrations measured in successive veneers
or subsamples as a function of distance from the centre of
the sample.
For a given metal or metalloid, a continuous decrease
with no plateau will indicate that exterior contamination has
8
Comments
Crag in and others
(1975)
Herron and others
(1977)
Petit and others
(1981)
Ng and Patterson
(J 981)
penetrated even to the centre of the sample so that the
concentrations measured in the central part of the sample
will give only an upper limit of the original concentration.
On the other hand, if the concentration clearly levels off to
a fixed value for several consecutive veneer layers or
subsamples, this indicates that the concentration measured in
this central part, after subtraction of procedural blanks, will
provide the investigator with a reliable estimate of the
original concentrations in Antarctic or Greenland snow and
ice.
Figure 1 shows the measured concentrations of Pb in
successive veneers from an ice-core section of age 5.5 ka
which was drilled electromechanically in a fluid-filled hole
at Camp Century, Greenland and in an ice core section of
age 2 ka which was thermally drilled in a non-fluid-filled
hole at Byrd station, Antarctica (Ng and Patterson 1981).
Enormous Pb contamination is observed on the outside of
the core in both cases. This outside value is however about
one order of magnitude higher for the core obtained in a
fluid-filled hole (0.8xI0- 6 g Pb g-l) than for the one
obtained in a non-fluid-filled hole (0.3xI0- 7 g Pb g-l). An
of
about
106-fold
and
104-fold,
overall
decrease
respectively, is observed when going from the outside to the
inside of these core sections. It is, however, impossible to
determine from these curves whether the concentrations
obtained for the central parts of the cores (1.6 and
2.2x I 0- 12 g Pb g-l, respectively) represent the original
concentrations in the ice or not. This could have been
determined only if the investigators had observed a plateau
for several consecutive veneer layers in the central parts of
the cores.
The same situation prevails for Cd, Zn and Cu in a
large size block (37x37x37 cm) of prehistoric blue ice
Bou/ron and Batifol:
10 6
I
in the Antarctic ice or not because the authors have
analyzed only a single sample from the centre to 12 cm
instead of analyzing several consecutive veneer samples.
Figure 3 shows the variations of the concentrations of
Pb when going from the outside to the centres of two
sections of a shallow snow-core which was hand-drilled
using an acid-cleaned all-plastic auger at stake D 55 in
East Antarctica (Boutron and Patterson 1983), and of one
section of a snow core which was hand-drilled with an
aluminium auger at Spaatz Island in the Antarctic Peninsula
(British Antarctic Survey 1982). In the D55 core section
from 8.56 to 8.68 m (Fig.3(A», the decrease of
concentrations is progressive, without any kind of plateau,
which shows that outside Pb contamination has intruded in
significant amounts to the centre of this sample. The rather
high Pb concentration (7.2xI0- 12 g Pb g-l) obtained for the
central part of this sample is probably unreliable . For the
D55 core section from 9.02 to 9.40 m (Fig.3(B» and for
the Spaatz Island core section (Fig.3(c» the situation is
radically different: a satisfying plateau of concentration is
observed for several consecutive veneer layers or subsamples;
it appears that no outside contamination has intruded to the
centre of these samples, so that the concentration values
observed in these central parts (3.2 and 9x10- 12 g Pb g-l,
respectively) will represent the original concentrations in the
Antarctic snow providing procedural blanks are subtracted.
I
I
Pb
10 5 -
,-
Ol
"-
'",.
Ol
0
10 4
-
II
I
A
c
o
I
I
o
~
10
3
I
8
-
<11
1 -
u
I
o
I
C
U
I
.0
Cl..
r...J
,,
10 2 -
"0
<11
~
I
I
::J
o
I
<11
:::!E
-
I
IJ)
I
10 1 f-
r~-...r
J
-
I
1----------
2.4.
Evaluation of the contamination introduced by the
decontamination procedure itself
Even if a clear plateau in the concentration profile is
obtained across the central part of a given sample, this does
not imply that the corresponding concentration value
represents the original concentration of Antarctic or
Greenland snow or ice. It is essential to assess the
contamination introduced by the decontamination procedure
itself and at each stage of the analytical process for each
metal or metalloid and then to subtract this procedural
blank from the plateau value.
To our knowledge, careful direct investigations of
procedural blanks have been performed only by Ng and
Patterson (I 981) and by Boutron and Patterson (I983) in
their Pb studies. To determine contamination introduced by
Pb during their chiselling procedure (see section 2.1), these
authors first performed careful analysis of the Pb content
(about 0.2xI0- 12 g Pb g-l) of the ultrapure water produced
in the ultraclean laboratory at the California Institute of
Technology. This water was then put in ultraclean
polyethylene bottles, sealed, and frozen into a configuration
r---.J
______ J
I
I
I
0 2 3
4
5
6
(cm)
Distance from center of core
Fig.1. Pb concentrations as a function of radius in (A) an
ice-core section, depth 949 m and age 5.5 ka,
electromechanically drilled in a fluid-filled hole at Camp
Century, Greenland, (B) an ice-core section, depth 342 m
and age 2.01 ka, thermally drilled in a non-fluid hole at
Byrd station, Antarctica. (From Ng and Patterson 1981.)
collected at an Antarctic coastal ablation site by Boutron
and others (1984) (Fig.2). These investigators observed a
sharp decrease in concentration of three metals when going
from the outside to 12 cm from the centre of the block
(2.6, 21.5 and 17.5 x 10- 12 g g-l for Cd, Cu and Zn,
respectively). It is difficult, however, to evaluate clearly
whether these values do represent the original concentrations
I
I
I
Zn
Cd
Ol
Cleaning ice cores for heavy metal analysis
Cu
-
600r-
"Ol
'"
'0
100-
1000-
IJ)
c
--
o 400f-
-
o
~
C
<11
U
C
o
SOOr-
u
"0
<11
-
200r-
~
t------'Jl
::J
IJ)
o
<11
~
o
1====I~J=------1
o
-
SOr-
-
10
20
Distance
o "--___IL-_ _-'
o
10
20
from center of block
o '--_---'1_ _---'
o
10
20
(cm)
Fig.2. Cd, Zn and Cu concentrations as a function of the distance from the centre of a
cubic block of surface prehistoric blue ice more than 12 ka in age collected in a
coastal ablation area at Cap Prudhomme, Antarctica. (From Boutron and others 1984.)
9
Boutron and Batifol:
Cleaning ice cores for heavy metal analysis
I
I
T
I
I
I
I
I
I
C
B
A
01
I
-
-
ci,60~
N
TO
'"oc
40~
-
"'0 20~
-
-"§
-
-
c
C1l
u
c
o
U
.0
0...
-
C1l
'-:J
'"o
C1l
~
o L-_..L-1---'...1_--,-I_---,
o
T
2340
I
I
I
2340
Distance from center of core
t
I
2
I
I
3
4
5
(cm)
Fig .3. Pb concentrations as a function of radius in (A) a snow-core section collected at
depth 8.56 to 8.68 m at stake D55, Antarctica, using an acid-cleaned all-plastic
hand-operated auger, (b) a snow-core section collected at depth 9.02 to 9.40 m. as
above, (C) a snow core collected at depth 2.04 to 1.09 m at Spaatz Island, AntarctJca,
using a clean aluminium auger. «A) and (B) are from Boutron and Patterson 1983, (C)
from British Antarctic Survey 1982.)
similar to the Antarctic or Greenland cores. The bottles
were cut using an acid-cleaned scalpel, and the artificial ice
cores so obtained were chiselled according to the procedure
described in section 2.1. The successive veneer layers and
the remammg inner core were then analyzed for Pb,
knowing beforehand how much Pb was in the artificial ice.
The chiselling procedure was found typically to introduce 50
to 400xlO- 12 g Pb (Ng and Patterson 1981, Boutron and
Patterson 1983), i.e. about 30% of the Pb content of a
typical Antarctic snow or ice sample.
These artificial samples were, of course, ice samples
only. It seems indeed almost impossible to make artificial
snow samples of known composItIon. As Boutron and
Patterson were studying Antarctic snow cores, they had to
make the implicit assumption that the procedural blank
obtained using artificial ice samples gave a good estimate of
the procedural blank for snow samples. This is a crucial
point which will obviously need to be substantiated in the
future.
3. RINSING DECONT AMINA TION TECHNIQUES
3.1
Description of the procedures
These procedures can of course be used only for ice
samples. They have been developed by Cragin, Herron,
Langway and their co-workers
for
their numerous
investigations of Pb, Cd, Zn, Cu and other elements in
various deep Greenland and Antarctic ice cores (Cragin and
others 1975, Herron and others 1977, Herron unpublished).
They were also used by LaViolette (unpublished) for his
study of Sb, Sn and other metals in a section of the deep
ice core from Camp Century, Greenland, and by Petit and
others (1981) for their study of Zn in the deep ice core
from Dome C, Antarctica.
The procedure used by all these investigators is similar
although there were slight variations dependent upon
whether the drilling hole was filled with a fluid or not. For
cores drilled without fluid, the procedure (Herron and
others 1973, Langway and others 1974, Petit and others
1981) involves removing the surface outer layer of the
ice-core section by running ultrapure water upon it inside a
temperate clean room, until a given fraction (20 to 60%
depending on the investigator) is removed. Upon completion
of this rinsing, the remaining inner core only is analyzed.
For cores obtained from fluid-filled holes, the procedure
(Herron and others 1973, LaViolette unpublished) first
involves several successive washings of the outer surface
10
with acetone; it then follows the process described above
for cores from non-fluid-filled holes.
3.2.
Checking the integrity of the central part of the
samples so obtained
It is necessary to assess clearly whether the central part
of the ice samples so obtained are free of contamination or
not. Very surprisingly, this has never been done for toxic
metals and metalloids by the various investigators who used
these rinsing procedures. To our knowledge, none of these
of
the
investigators
have
studied
the
variations
concentrations of toxic metals and metalloids in the
remaining inner core as a function of the percentage of ice
melted off. Such curves have been obtained only for Na,
Mg, K and Ca by Ragone and Finelli (1972) and by
Ragone and others (1972), and for S04' N0 3 , NH 4, Cl, Na
and K by Legrand and others (1984).
It is therefore impossible to evaluate the efficiency of
these rinsing decontamination techniques, so that all data on
toxic metals and metalloids obtained from the analysis of
samples decontaminated in this way must be considered
suspect. All these data will need to be validated in the
future by careful investigation of the efficiency of the
rinsing procedures.
For Pb, however, there is now already clear evidence
that the deep ice-core sections, which were decontaminated
by means of the rinsing procedure of Cragin and others
(J 975), were still highly contaminated by up to one order
of magnitude. This was convincingly demonstrated by Ng
and Patterson (I 981) who carefully studied the radial
contamination characteristics of several Antarctic ice-core
sections which were analogues of part of the sections of
Greenland ice core studied by Cragin, Herron and
co-workers (Herron and others 1977).
3.3.
Evaluation of the contamination introduced by
rinsing procedures
No investigator has so far tried to determine the
contamination introduced for toxic metals and metalloids by
rinsing procedures. This will obviously need to be done in
the future by processing artificial ice-core sections made in
the
laboratory
by
freezing
ultrapure
water
whose
composition is accurately known.
4.
CONCLUSION
It appears that much work still remains to be done in
order to assess clearly the efficiency of the various
Boutron and Bati/ol:
decontamination procedures which have been developed for
the analysis of toxic metals and metalloids in Antarctic and
Greenland snow and ice samples. Both mechanical and
rinsing procedures need to be fully assessed, but it is our
feeling that the rinsing procedures will probably prove to
be inefficient for toxic metals and metalloids, as already
shown for Pb. The main efforts should probably be devoted
towards improving chiselling procedures or direct mechanical
subsampling procedures, and evaluating their efficiency not
only for Pb, as already done by Patterson and co-workers,
but also for the other toxic metals and metalloids.
Only when such a careful evaluation has been made
will it be possible to obtain reliable data on the
concentrations of toxic metals and metalloids in Antarctic
and Greenland snow and ice. This will be a great
achievement for then it will be possible to obtain reliable
historical records for these elements in the remote polar
hemispheres and to investigate their
areas of both
past natural cycles on a global scale.
REFERENCES
Appelquist H, lensen K 0, Sevel T, Hammer C 1978
Mercury in the Greenland ice sheet. Nature 273(5664):
657-659
Boutron C 1979 Reduction of contamination problems in
sampling of Antarctic snows for trace element analysis.
Analytica Chimica Acta 106: 127-130
Boutron C 1982 Atmospheric trace metals in the snow
layers deposited at the South Pole from 1928 to 1977.
Atmospheric Environment 16( I 0): 2451-2459
Boutron C In press Atmospheric toxic metals and metalloids
in the snow and ice layers deposited in Greenland and
Antarctica from prehistoric times to present. In Davidson
C I, Nriagu J 0 (eds) Toxic metals in the air. New
York, Wiley (Advances in Environmental Science and
Technology Series)
Boutron C, Patterson C C 1983 The occurrence of lead in
Antarctic recent snow, firn deposited over the last two
centuries and prehistoric ice. Geochimica et Cosmochimica
Acta 47: 1355-1368
Boutron C, Leclerc M, Risler N In press Atmospheric trace
elements in Antarctic prehistoric ice collected at a coastal
ablation area. Atmospheric Environment 18(9): 1947-1953
British Antarctic Survey 1982 British Antarctic Survey
Annual Report. 1981-82. Cambridge, British Antarctic
Survey
Cragin J H, Herron M M, Langway C C Jr 1975 The
chemistry of 700 years of precIpItation at Dye 3,
Greenland. CRREL Research Report 341
Hansen B L, Langway C C Jr 1966 Deep core drilling in
ice and core analysis at Camp Century, Greenland,
1961-1966. Antarctic Journal of the United States 1(5):
207-208
Herron M M Unpublished The impact of volcanism on the
chemical composition of Greenland ice sheet precIpItation.
(PhD thesis, State University of New York at Buffalo,
1980)
Herron M M, Cragin J H, Langway C C Jr 1973 Improved
procedures for the removal of surface contaminants from
ice cores for chemical analysis. CRREL Technical Note
Herron M M, Langway C C Jr, Weiss H V, Cragin J H
1977 Atmospheric trace elements and sulphate in the
Greenland ice sheet. Geochimica et Cosmochimica Acta
41(7): 915-920
Landy M P, Peel D A 1981 Short term fluctuations in
heavy metal concentrations in Antarctic snow. Nature
291(5811): 144-146
Langway C C Jr, Herron M M, Crag in J H 1974 Chemical
profile of the Ross Ice Shelf at Little America V,
Antarctica. Journal of Glaciology 13(69): 431-435
LaViolette P A Unpublished Galactic explosions, cosmic
dust invasions, and climatic change. Part I. (PhD thesis,
Portland State University, 1983)
Legrand M, De Angelis M, Delmas R J 1984 Ion
chromatographic determination of common ions at ultra
trace levels in Antarctic snow and ice. Analytica Chimica
Acta 156: 181-192
Cleaning ice cores for heavy metal analysis
Murozumi M, Chow T J, Patterson C C 1969 Chemical
concentrations of pollutant lead aerosols, terrestrial dusts
and sea salts in Greenland and Antarctic snow strata.
Geochimica et Cosmochimica Acta 33(10): 1247-1294
Ng A, Patterson C C 1981 Natural concentrations of lead in
and
Antarctic
ice.
Geochimica
et
ancient
Arctic
Cosmochimica Acta 45(1 I): 2109-2121
Patterson C C, Settle D M 1976 The reduction of orders of
magnitude errors in lead analysis of biological materials
and natural waters by evaluating and controlling the
extent and sources of industrial lead contamination
introduced during sample collection and analysis. In La
Fleur P (ed) Accuracy in trace analysis.
Washington,
DC, National Bureau of Standards: 321-351 (Special
Publication 42)
Peel D A, Wolff E W 1982 Recent variations in heavy
metal concentrations in firn and air from the Antarctic
Peninsula. Annals of Glaciology 3: 255-259
Petit J-R, Briat M, Royer A 1981 Ice age aerosol content
from East Antarctic ice core samples and past wind
strength. Nature 293(5831): 391-394
Ragone S E, Finelli R V 1972 Procedures for removing
surface contaminants from deep ice cores. CRREL Special
Report 167
Ragone S E, Finelli R V, Leung S, Wolf C 1972 Cationic
analysis of the Camp Century, Greenland, ice core.
CRREL Special Report 179
Wolff E W, Peel D A 1985[a] Closer to a true value for
heavy metal concentrations in recent Antarctic snow by
improved contamination control. Annals 0/ Glaciology 7:
61-69
Wolff E W, Peel D A 1985[b1 The record of global
poll u tion in polar ice and snow. Nature 313(6003):
535-540
II